1. Field of the Invention
This invention relates to a method of forming a superconductor pattern. More particularly, the present invention relates to a method of forming a superconductor pattern without lowering a critical current density.
2. Description of the Related Art
It has long been known that many metals and inorganic compounds such as molybdenum carbide (MoC), niobium germanium (Nb.sub.3 Ge), etc., exhibit a superconductivity at ultra-low temperatures, but the superconduction transition point (Tc) of these inorganic compounds is low, and is at most 10K for metals and 23.5K for Nb.sub.3 Ge.
The discovery of high temperature superconduction of oxide ceramics of lanthanum-barium-copper-oxygen (La--Ba--Cu--O) system, in 1986, led to intensive studies into and the development of high temperature superconductive ceramics having a critical temperature Tc above the boiling point (77.2K) of liquid nitrogen (N.sub.2), such as a Bi system consisting of bismuth-strontium-ca-cium-copper-oxygen (Bi--Sr--Ca--Cu'O), a Y system consisting of yttrium-barium-copper-oxygen (Y--Ba--Cu--O), and a Tl system consisting of thallium-barium-calcium-copper-oxygen (Tl--Ba--Ca--Cu--O), etc.
Semiconductor devices constituting the principal portion of data processing units are made of simple substance semiconductors, typified by silicon (Si), or compound semiconductors typified by gallium arsenide (GaAs), due to the necessity for quickly processing large quantities of data, and integrated circuitry such as ICs and LSIs are in practical application.
A conductor pattern consisting the integrated circuit is formed by depositing a thin metal film, such as of gold (Au), aluminum (Al), etc., on a semiconductor, substrate (wafer), and forming a fine pattern thereof by photolithography, according to the prior art.
Data processing, however, could be made at a far higher speed and a much lower loss than in the prior art if a high temperature superconductor were to be used, in place of a metal, as the material of the conductor pattern, and the transmission of signals effected by utilizing superconduction and employing liquid nitrogen (N.sub.2) as a cooling medium.
Various oxide superconductors are known, as described above, and in all of such superconductors, oxides assuming a two-dimensional structure are laminated to form a unit lattice, and the repetition of this structure forms a superconductor thin film having a predetermined thickness.
It is customary to use a material having a low reactivity with the superconductor, such as magnesia (MgO), strontium titanate (SrTiO.sub.3), etc., as a substrate, and to grow a superconductor thin film by vacuum deposition, sputtering, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), or the like.
The following explanation relates to the case wherein the film of a Bi system superconductor is formed by vacuum deposition on a MgO substrate. The MgO substrate is first positioned in an ion plating apparatus and, while the substrate is heated, the interior of a deposition chamber of the apparatus is maintained in an oxygen (O.sub.2) atmosphere of about 1.times.10.sup.-4 Torr. Power is then supplied to a radio frequency coil and while an oxygen (O) plasma is generated in a discharge region, a vacuum deposition of Bi, Sr, Cu, Ca, Cu, Sr and Bi is sequentially carried out, in that order. Thereafter, the reactive vacuum deposition is effected and the deposited metal atoms are oxidized, whereby these oxides are built up and provide an oxide superconductor thin film.
FIG. 7 shows the lattice structure of the Bi system superconductor formed in this manner. The lattice is oriented with the C axis directed upward, perovskite layers of a 1/2lattice exist vertically through (BiO).sub.2 layers, and the BiO layers are weakly bonded to one another by the van der Waals force. Therefore, a cleavage of the superconductor occurs at this position, and due to the existence of this cleavage, the electric conductivity in the longitudinal direction (C-axis direction) becomes low.
Next, to form a conductor pattern by patterning the resulting superconductor thin film, electrodes must be formed at both ends of the pattern.
A conventional method of forming a superconductor pattern comprises patterning a superconductor thin film 2 (FIG. 2B) on a substrate 1 (FIG. 2A), forming electrodes 3 on respective, opposite end portions of this superconductor thin film 2, and forming a superconductor pattern (FIGS. 2A to 2C), as shown in FIG. 2. In the conventional superconductor pattern (hereinafter referred to as the "Prior Art Example 1") formed in this manner, however, when an voltage is applied to the electrodes 3, a current flows mainly along the surface of the superconductor thin film 2, but part of this current enters the inside thereof. Since electric resistance is high, however, the characteristics of the superconductor are lowered and the critical current density drops (see FIG. 4).
Another conventional method comprises forming a superconductor thin film 12 on a substrate 1 (FIGS. 3A and 3B), patterning this thin film 12 by photolithography, and forming electrodes 13 at both end portions of the thin film 13, as shown in FIG. 3C (hereinafter referred to as the "Prior Art Example 2"). This method, however, has a problem in that the film quality is deteriorated during the etching step, and thus the critical current density of the superconductor drops (see FIG. 4).
To form a conductor pattern by patterning a superconductor thin film, electrodes must be formed at both ends of this pattern, but the superconductor has a laminar structure and the electrical conductivity in the c axis thereof is lower than in the a and b axes. Accordingly, the conventional methods of forming the electrodes cannot pass a current having a uniform current density through the section of the film, and this causes the drop of the critical current density.